The invention relates to an apparatus and a process for degassing plastic materials, in particular high-molecular polycarbonate solutions, by means of a double-shaft extruder, which comprises shafts rotating in the same direction and meshing with one another.
As a result of heightened environmental awareness, increasing demands regarding the removal of volatile components from plastic materials are being placed on the plastics processing industry. This applies particularly with regard to the use of plastic materials in the food sector. The volatile components in the extruded plastic material mostly may not exceed 0.2% by weight and are required to be removed in order to improve the product quality, especially the optical properties. This applies particularly to products made from high-molecular polycarbonate solutions containing chlorobenzene and methylene dichloride as volatile components. In the plastics industry, the removal of volatile components from polymers is known as degassing. Various apparatuses are used for degassing, in particular single- and twin-screw extruders.
In the case of degassing by means of single- and twin-screw extruders, a distinction is made between forward and backward degassing as well as single and multiple degassing. With forward degassing the vent opening of the extruder is disposedxe2x80x94in feed direction of the screwxe2x80x94downstream of the intake of the extruder, whereas with backward degassing it is disposedxe2x80x94in feed direction of the screwxe2x80x94upstream of the intake. Degassing extruders are also used, in which both forward and backward degassing are effected. Depending on the desired residual content of volatile components, single-stage or multi-stage degassing is effected. The number of degassing stages cannot however be increased indefinitely because this entails an increase in the manufacturing cost of the product and frequently also a decrease in product quality. The product quality in said case is particularly dependent upon the processing temperature or the temperature rise as well as the retention lime in the extruder of the plastic material to be degassed.
The object of the invention is to provide an apparatus and a process of the type described initially, which lead to an economical improvement of product quality, and indeed in particular enable manufacture of polycarbonate of a particularly high transmittancy.
Said object is achieved according to the invention in that the shafts (30 and 33) of the extruder are designed with a double lead (19) in the degassing zone and with a triple lead (21) in the pressure build-up zone of the extruder and the extruder has an L:D ratio smaller than/equal to 40, wherein L is the respective screw length and D the respective screw diameter.